This invention relates to methods of producing oriented films of poly-L-lactic acid and/or poly-D-lactic acid, as well as to oriented films thus produced.
Poly-L-lactic acid and poly-D-lactic acid are aliphatic polyesters and are considered useful biodegradable materials because of their hydrolytic property. Although they are known materials, the technology of making them into a film has not been established yet because of their crystallizing property and rigidity. In general, when a polymeric material is stretched, its molecular chains and crystallites become oriented in the direction of the stretching such that its mechanical strength is improved and changes occur in its physical properties. Thus, it may be expected with poly-L-lactic acid and poly-D-lactic acid, too, that improvement in mechanical strength and changes in physical properties will result if they are subjected to a stretching process. Polylactic acids, and poly-L-lactic acids in particular, however, have a rigid molecular structure providing them with a high crystalline, and it has been difficult to improve their physical properties by stretching.